The use of radio signals to estimate the location of a wireless device or node is known. For example, a Global Positioning System (GPS) receiver obtains location information by triangulating its position relative to four satellites that transmit radio signals. The GPS receiver estimates the distance between each satellite based on the time it takes for the radio signals to travel from the satellite to the receiver. Signal propagation time is assessed by determining the time shift required to synchronize the pseudo-random signal transmitted by the satellite and the signal received at the GPS receiver. Although triangulation only requires distance measurements from three points, an additional distance measurement from a fourth satellite is used for error correction.
The distance between a wireless transmitter and a receiver can also be estimated based on the strength of the received signal, or more accurately the observed attenuation of the radio signal. Signal attenuation refers to the weakening of a signal over its path of travel due to various factors like terrain, obstructions and environmental conditions. Generally speaking, the magnitude or power of a radio signal weakens as it travels from its source. The attenuation undergone by an electromagnetic wave in transit between a transmitter and a receiver is referred to as path loss. Path loss may be due to many effects such as free-space loss, refraction, reflection, aperture-medium coupling loss, and absorption.
Path loss is mathematically characterized as a path loss model that includes a path loss exponent, which is a parameter defining the attenuation of a radio signal over a unit distance. Accordingly, the path loss model can be used to provide an estimate of the distance between a radio transmitter and receiver, given the power at which the signal was transmitted, and the power at which the signal was received. More complicated path loss models can include additional parameters such as wall loss exponents which characterizes the attenuation of radio signals propagating through walls and other physical barriers.
In prior art wireless node location mechanisms, path loss exponents are statically defined values. Generally, path loss exponents are values derived from heuristic evaluations of path loss typically conducted in a generic, simulated test environment. The static path loss exponent values employed to estimate distance are an average observed path loss and, essentially represent a compromise, since path loss generally varies with the static and dynamic elements of a physical space. That is, actual path loss varies with the attributes of a physical space, such as the number and location of walls, doors and windows. While it is possible to compute path loss exponents for a given physical space in which a wireless node location mechanism is deployed, this generally requires expensive site surveys and analysis. In addition, even these path loss exponents are nevertheless static values. Path loss exponents will vary depending on the location of wireless nodes, as well as changes to the attributes of the physical space, such as new or removed walls, dividers, windows, doors, and even plants. In addition, actual path loss will also vary depending on the location of other physical objects that often change, such as people, equipment, etc, as well as whether doors or windows are open or closed. The difference between actual path loss and the statically defined path loss exponents used to estimate distance adversely affects the accuracy of wireless node location mechanisms.
In light of the foregoing, a need in the art exists for a wireless node location mechanism that accounts for changes to a surrounding physical environment that affect the propagation of radio signals. Embodiments of the present invention substantially fulfill this need.